Page 480 - Injection Molding Advanced Troubleshooting Guide
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480 47 Weld Lines
Ultimately this problem was never completely resolved. Different-sized
flakes with an aspect ratio of closer to 1 : 1 (height to width) were used,
which helped to improve the defect. Metallic flakes will create struggles with
weld lines. If there is interest in a metallic-filled material make sure that
everyone sees molded samples with weld lines before deciding on a color.
Figure 47.6 Weld line with metallic pigment
47.3.4.2 Material: Viscosity
A lower-viscosity material will provide a better chance for a weld line to come
together and allow entanglement of the plastic molecules at the weld line. Lower
material viscosity will allow easier mold filling and better transfer of pressure
across the cavity. A higher melt index material may result in better weld lines.
Remember that increased melt index often means reduced physical properties.
Be sure to validate any material change through the customer and all applicable
testing.
With hygroscopic materials watch for viscosity shifts from moisture content. A
change in moisture content can lead to a viscosity difference that impacts the for-
mation of weld lines.
